This invention relates to rotary printing of images on a series of sheets passed through a printing machine, including at least one conveyor, while maintaining registry of the sheets with respect to rotary cylinders positioned both upstream and downstream of the conveyor.
In the art of rotary printing, those machines which print on individual, separated sheets passing successively through the machine inherently have a critical problem of registration control which is not present in web-type printing machines. That is, in order to produce clean and sharp images, the linear position of each sheet must be in perfect registry with the angular positions of the rotary cylinders positioned both upstream and downstream of the conveyor, including registry with the rotary cylinder which is in engagement with the sheet at that precise time. Otherwise, the printed images on the sheets become blurred and may be totally unacceptable.
U.S. Pat. No. 5,383,392 discloses a registration control system which has been a significant advance in the rotary printing art. This control system is designed for use when several of the rotary cylinders are driven by separate servo motors with each of the servo motors being individually controlled by the control system. However, as is well known in the rotary printing art, servo motors are extremely expensive, and no solution to the critical registration problem is known for rotary printing machines in which the various rotary cylinders are driven through a gear train by a single drive motor. This type of gear drive system with a single motor is much less costly than a plurality of independent servo motors; however, the critical problem of inaccurate registration because of the unavoidable backlash in the gear train has never been satisfactorily solved.
The present invention solves this long-standing problem by sensing the angular positions of the cylinders both upstream and downstream of the conveyor, and adjusting the speed of the conveyor in accordance with these sensed conditions, as well as, the sensed linear position of each sheet.